1. Field of the Invention
The present invention relates to a method for measuring the flow rate of an exhaust gas.
2. Description of the Related Art
In a conventional method for measuring the flow rate of fluid passing through a pipe, the flow rate is determined by the difference between the pressure in the pipe and the external pressure. This method is applied in, for example, a spin coating apparatus used in manufacturing a semiconductor device, to measure the rate at which exhaust gas flows through a pipe. More specifically, a semiconductor wafer is placed on a supporting stage provided in a coater cup. While the stage is being rotated at a high speed, photoresist is dripped onto the wafer, spread thinly by centrifugal force, and coated thereon. In this process, the surplus photoresist diffuses into the surrounding gas like mist. For this reason, to discharge the gas containing the surplus photoresist, a gas-discharging mechanism is used. The gas-discharging mechanism has a pressure sensor such as a pitot tube to measure the flow rate of the gas discharged through the exhaust pipe. A differential manometer is connected to the pitot tube, and measures the difference between the atmospheric pressure and the pressure of the exhaust gas in the pipe, from which the flow velocity of the exhaust gas is determined. The flow rate is obtained based on this velocity.
According to the above-described conventional method, impurities in the exhaust gas, such as the mist-like photoresist, adhere to and clog the inlet of the pitot tube. As a result, the flow rate cannot be measured accurately.